The invention relates to a process for the preparation of mouldings, especially optical lenses and in particular contact lenses, to a corresponding device for the preparation of mouldings, and to the mouldings that have been prepared or are obtainable by the process or using the device, especially optical lenses and in particular contact lenses, in accordance with the preamble of each independent patent claim.
Contact lenses, which are to be manufactured economically in large unit numbers, are preferably manufactured by the so-called mould or full-mould process. In these processes, the lenses are manufactured into their final shape between two moulds, so that there is no need to subsequently finish the surfaces of the lenses, nor to finish the edges. Such moulds consist of a female mould half and a male mould half, the cavity being formed between the two mould halves defining the shape of the moulding. Mould processes are described for example in PCT application No. WO/87/04390 or in European patent application EP-A-0 367 513.
In these known mould processes, the geometry of the contact lenses to be manufactured is defined by the mould cavity. The edge of the contact lens is likewise formed by the mould which normally consists of two mould halves. The geometry of the edge is defined by the contour of the two mould halves in the area in which they touch one another.
To prepare a contact lens, first of all a certain amount of a flowable starting material is placed in the female mould half. Afterwards, the mould is closed by placing the male mould half thereon. Normally, a surplus of starting material is used, so that, when the mould is closed, the excess amount is expelled into an overflow area outwardly adjacent to the mould cavity. The subsequent polymerisation or crosslinking of the starting material takes place by radiation with UV light, or by heat action, or by another non-thermal method. Both the starting material in the mould cavity and the excess material in the overflow area are thereby hardened. In order to obtain error-free separation of the contact lens from the excess material, a good seal or expulsion of the excess material must be achieved in the contact zone of the two mould halves. Only in this way can error-free contact lens edges be obtained.
The contact lenses produced in this manner are moulded parts having little mechanical stability and a water content of more than 60% by weight. After manufacture, the lens is inspected, then packed and subjected to heat sterilisation at 121xc2x0 C. in an autoclave.
The materials used for these moulds are preferably plastics, e.g. polypropylene. The moulds are produced by injection moulding and are only used once. This is because, among other things, the moulds are partially contaminated by the surplus material, are damaged when the contact lens is separated or are irreversibly deformed in partial areas when the mould is closed. In particular, because of the quality requirements of the contact lenses edges, the moulds are only used once, since a certain amount of deformation of the moulds at the area of their edge cannot be excluded with certainty.
In U.S. Pat. No. 5,508,317, a new contact lens material is described, which represents an important improvement in the chemistry of polymerisable starting materials for the manufacture of contact lenses. The patent discloses a water-soluble composition of a prepolymer, which is filled into the mould cavity and then crosslinked photochemically. Since the prepolymer has several crosslinkable groups, the crosslinking is characterised by its high quality, so that a finished lens of optical quality can be produced within a few seconds, without the necessity for subsequent extraction or reworking steps. Owing to the improved chemistry of the starting material as illustrated in the patent, contact lenses can be produced at considerably lower cost, so that in this way it is possible to produce disposable lenses that are used only once.
EP-A-0 637 490 describes a process by means of which a further improvement may be obtained in the preparation process of contact lenses with the prepolymer described in U.S. Pat. No. 5,508,317. Here, the material is filled into a mould comprising two halves, whereby the two mould halves do not touch, but a thin circular gap is located between them. The gap is linked to the mould cavity, so that surplus lens material can flow away into the gap. Crosslinking of the prepolymer takes place by radiation especially with UV light, whereby radiation is restricted to the mould cavity by a chromium mask. In this way, only the material in the mould cavity is crosslinked, so that there is high reproducibility of the edges of the lens without closing the two mould halves.
In this process, instead of the polypropylene moulds that may be used only once, reusable quartz/glass moulds are used. Because of the water-soluble basic chemistry, after a lens has been produced, the uncrosslinked prepolymer and other residues can be removed from the moulds rapidly and effectively and dried in the air. In addition, quartz is notable for its good UV permeability and is very hard and refractory.
When using a prefunctionalised PVA (polyvinyl alcohol) polymer as lens material, moreover, important material properties of quartz casting moulds are the excellent replication of the surface geometry, as well as the transparency of the material, so that it is possible to provide visual control of the lens in the mould.
However, moulds made from quartz or glass are very expensive to produce, so that owing to the high costs, the moulds ought to have quite long service life in order to ensure that the process is economical. Therefore, for economic reasons, only a limited number of variants can be realised, for example in respect of the dioptre number.
In order to solve this problem, the intended objective is to replace at least one of the two mould halves, especially the female mould half, with a mould half consisting of plastic, and thus to employ one mould half made of plastic in combination with one mould half made of quartz or glass.
In the plastic moulds, especially polypropylene moulds, which were previously known in the prior art, there was however the problem that the lenses produced with these moulds had a slippery surface. This is because the slipperiness is caused by oxygen, which leads to an inhibition of the crosslinking reaction at the surface of the lens, this becoming apparent macroscopically in a xe2x80x9cslipperinessxe2x80x9d of the lens. It is assumed that oxygen, which is already present at the surface of the casting mould or migrates to the surface of the mould during the polymerisation process, inhibits polymerisation of the lens material at the lens surface. Thus, the interface between the mould and the lens material appears to be crucial.
In order to avoid this problem, it is known from EP-A-0 687 550 that the plastic mould halves can be degassed and then transported and filled in a nitrogen atmosphere. This however requires a technical process that is very complex and in addition is very cost-intensive.
WO-A-96/11782 describes a process operating under an inert gas atmosphere and/or one using moulds in which the oxygen contained therein has been drawn off completely by means of treatment with a vacuum or with an inert gas, thus producing xe2x80x9cnon-slipperyxe2x80x9d lenses. It also describes that the oxygen in the material from which the lenses are produced is completely drawn off prior to lens manufacture. However, the cost of these additional process steps is very high and the process steps are very time-consuming.
It is the aim of the present invention to further develop a process and a device of the generic kind and to improve them in such a way that it is possible to use mould halves of plastic in conjunction with mould halves of quartz or glass, without the above-mentioned difficulties and problems arising. In particular, the plastic mould halves should be reusable, and any burr or web formation on the finished contact lenses should be avoided, so that the rejection rate of the contact lenses is very low. In addition, optical monitoring of the lens through the casting mould should be possible.
The invention solves the problem by means of the features indicated in claim 1 and claim 11. As far as further essential developments of the process according to the invention and of the device according to the invention are concerned, reference is made to the dependent claims.
Through the choice of polymers for the production of plastic mould halves, which have very low oxygen permeability, the oxygen is prevented to a great extent from reaching the cavity during polymerisation, and thus prevented from making contact with the starting material for the lenses. The idea behind this is that the oxygen which impairs polymerisation or crosslinking of the lens surface originates from the casting mould with which the lens material comes into contact. It could be shown that when using polymers for the mould, which have only slight oxygen permeability, lenses having no surface slipperiness can be produced. It is thus possible to manufacture lenses in an atmospheric environment whilst simultaneously using plastic mould halves. Since it is not necessary to produce the lenses in a nitrogen atmosphere, production costs may be substantially reduced.